Just to clarify, the attached screenshot is what the current login page looks like on mobile. I believe what Adam is asking for is a more prominent button/call to action to create an account, rather than a plain link.
There's a UX quality tradeoff involved in this decision. We don't (yet) have any onboarding mechanisms in place for new users on mobile. When we showed the big blue "add an image to this article" button to readers and invited them to log in or create an account, we got a flood of new users creating accounts on mobile... and a corresponding flood of selfies uploaded via mobile web that had to be deleted from Commons. The lesson there was that brand new users who don't already have a Wikimedia account don't necessarily understand how to contribute constructively, and without a well-thought-out first time user experience/onboarding, we're just dumping those people into a meat grinder.
We're currently building out more first-time user education, so eventually we might consider making the create account link bigger and inviting more brand-new users into our ecosystem. But until we really feel confident that we've nailed it, it's honestly probably better for those users and for our editing community to make login the primary call to action, even if that means losing some potential new contributors.